1. Field of the Invention
The present invention relates to a method for determining a beamlet position in a charged particle multi-beamlet exposure apparatus. The present invention further relates to a method for determining a distance between two beamlets in a charged particle multi-beamlet exposure apparatus. Finally, the present invention relates to a computer readable medium for performing, when executed by a processor, one of the abovementioned methods.
2. Description of the Related Art
In order to transfer a pattern onto the target surface, the controllable blocking of beamlets in combination with their movement over the target surface is performed in accordance with modulation information. An example of a multiple charged-particle beamlet lithography system is described in U.S. Pat. No. 6,958,804, which disclosure is herewith incorporated by reference in its entirety.
Such lithography systems can have very large numbers of beamlets, i.e. in the order of 10,000 or higher, for example 13,000. Future designs even envisage numbers in the order of 1,000,000 beamlets. It is a general aim for current electron beam lithography systems to be able to pattern a target surface in high-resolution, with some applications being capable of imaging patterns with a critical dimension of well below 100 nm feature sizes.
For such multiple beamlet, high-resolution lithography systems to be commercially viable they need to meet low error margins. Therefore it is important that the position of each one of the charged particle beamlets is precisely known and controlled. Due to various circumstances, such as manufacturing tolerances and thermal drift, such positions may however deviate from their expected and desired positions, which may render these deviating beamlets invalid for accurate patterning.
In conventional lithography systems, the position of each beamlet is determined by frequent measurement of the beamlet position. With knowledge of the beamlet position the beamlet can be shifted to the correct position.
Known beamlet position calibration methods generally comprise at least three steps: a measuring step in which the position of the beamlet is measured, a calculating step in which the measured position of the beamlet is compared to the desired expected position of that beamlet, and a compensation step in which the difference between the measured position and the desired position is compensated for. Compensation may be performed either in the software or in the hardware of the lithography system.
It is desirable to determine beamlet position during operation of a lithography system to allow for early position calibration to improve the target surface patterning accuracy. In order to limit negative effects on throughput, i.e. the number of target surfaces that can be patterned within a predetermined period of time, it is desirable that the method of measuring the position of the charged particle beamlets can be carried out within a limited period of time without sacrificing accuracy.
In particular, in view of the continuously increasing demands of the industry regarding small dimensions without loss of throughput, there is a need to provide more accurate devices and/or techniques for measurement of beamlet properties in lithography systems, particularly in lithography machines comprising a large number of charged-particle beamlets that are designed to offer a high throughput. A higher accuracy is advantageous for increasing the resolution of a lithography machine.
In particular it is favourable when using stitching, a technique where two beams write on the same area of the wafer, for example to correct for writing failures. The beam separation needs to be known with nanometer precision for this technique.
Furthermore, there is a need to be aware of the absolute position of the beamlets. In particular, such knowledge of absolute position is favorable to improve the accuracy of overlay, i.e. a measure of the alignment accuracy of successive layers or features provided by multiple processes within the same layer with respect to a previously exposed or otherwise patterned layer.
US-patent application 2007/0057204 describes a method for determining the position of charged particle beams. In this method, the position of each charged particle beam within a plurality of charged particle beams is measured by using a converter for converting a charged particle beam into a light beam, and a photon receptor. Optionally, a blocking element is provided to the surface of the converter.
International application WO2012/062931 describes a method for determining a distance between two beamlets in a multi-beamlet exposure apparatus. In this method, a converter is used provided with a sensor surface area provided with a two-dimensional pattern of beamlet blocking and non-blocking regions.
In the techniques used in abovementioned patent documents is based on analysis of the light output related to an impinging charged particle beamlet. If scanning is performed, the light output change as a result of moving the charged particle beamlet from a blocking region towards a non-blocking region, or vice versa, is analyzed.
Although very useful for many applications, the accuracy of this technique depends on the spot size of the beamlet as compared to the dimensions of the blocking feature and/or blocking/non-blocking pattern on the converter surface, i.e. the feature size. If the spot size is much smaller than the feature size, finding the correct position of a beamlet is time-consuming. On the other hand, if the spot size of the beamlet is large compared to the feature size, the position is difficult to find because it will be difficult to fit the measurement results.